The study has established that rabbit blastocysts can synthesize Estradiol (E2) from testosterone. There are also strong indications that mouse blastocysts have aromatase activities but the estrogens formed are probably metabolized further, therefore, only little amounts of E2 can be detected. Thus it is urgent to determine whether or not mouse blastocysts can metabolize E2. Because of intimate relationship between the blastocyst and the endometrium, both should be examined closely for their steroid metabolizing activities; we will start with their aromatase activities. Mouse and rat blastocyts differ not only in their morphology at implantation but also their concanavalin A binding capacities. Preliminary data suggest that mouse and rat blastocysts may be also different in their steroid metabolizing activities, both quantitatively and qualitatively. Thus, we propose the following studies: a. To determine whether the mouse blastocyst can metabolize estradiol into other metabolites: b. To quantitate the estrogen production from tritiated testosterone by the mouse blastocysts, using radioimmunoassay. c. To determine if rat blastocysts are capable of producing estrogen from testosterone. d. To determine whether the mouse and rat endometrium possess aromatase activities. e. To determine the control mechanism of estrogen synthesis in the rabbit blastocyst: could HCG, prostaglandins and histamine affect it?